This invention relates to a method of joining aluminum surfaces by brazing.
It is known to join aluminum components by disposing an aluminum brazing alloy between or adjacent the component surfaces to be joined, and heating the brazing alloy and the joining surfaces in appropriately assembled fashion to a temperature (brazing temperature) at which the brazing alloy melts while the components remain unmelted. Upon subsequent cooling, the brazing alloy forms a fillet or joint that bonds the joining surfaces of the components. For assured selective melting of only the brazing alloy in the heating step, it is commonly preferred that the melting point of the brazing alloy be at least 30.degree. to 40.degree. C. lower than that of the metal of the components. An example of a suitable aluminum brazing alloy is an Al-Si eutectic composition, which melts at about 577.degree. C.
It is common practice to pre-clad at least one of the surfaces being joined with a layer of aluminum brazing alloy. Such pre-clad articles, generally known as brazing sheet, are relatively costly, and in many instances it is preferred to provide the brazing alloy in some form other than cladding. One known alternative is to apply the brazing alloy to or adjacent to one or both joining surfaces in powdered or particulate form carried in a suitable liquid or paste-like vehicle.
(Cooke) In U.S. Pat. No. 3,971,501 there is described a method of joining aluminum surfaces with an aluminum brazing alloy in which a mixture of the aluminum brazing alloy in powdered form and a powdered flux is coated on the surfaces to be joined. The coating is then dried and the surfaces are then heated to a brazing temperature whereby the brazing is completed. This procedure, like other known systems, utilizes a brazing alloy.
Although fluxless brazing procedures have been devised, their use is limited because of economic and other considerations arising from the special conditions and equipment required for successful practice of such procedures. It is, therefore, generally necessary to employ a flux in brazing aluminum, to remove the oxide ordinarily present on exposed metal surfaces at the location of the joint, as well as to promote flow of molten brazing alloy during the heating step. The material used as a flux must be capable of acting as a flux to dissolve and/or otherwise remove the metal oxides at the brazing temperatures while remaining essentially inert with respect to the aluminum at such temperatures. Since fluxes are usually reactive only when at least partially molten, fluxes for aluminum brazing should as a practical matter be partly or wholly molten at brazing temperatures, e.g. at temperatures not substantially higher and preferably lower than 577.degree. C.
Flux materials heretofore commercially employed in brazing aluminum have commonly been mixtures of predominantly chloride salts, with minor additives of fluoride in some cases. A recent highly successful flux for brazing aluminum is one sold under the trade mark NOCOLOK.RTM., which is essentially a potassium fluoaluminate.
It is the object of the present invention to provide an improved method of joining aluminum surfaces by brazing without the prior formation of a brazing-alloy cladding on the surfaces to be brazed.